Sadiq Khan is extending London's congestion charge to all zero-emission vehicles from the end of next year.
The move, which will extend the £15-a-day tax on motoring to battery-powered electric vehicles from Christmas Day 2025, was widely condemned on Tuesday.
Mark Harper, the Transport Secretary, said the plans showed that Labour wanted to use motorists as "cash cows" if the party wins this week's general election.
The AA described the scrapping of the green exemption as "a turkey bone in the throat" for owners of electric cars wanting to drive them in the capital.
Mr Khan's office declined to comment when approached by The Telegraph.
The Mayor of London has spoken extensively over the years about his personal drive to improve London's air quality by imposing swingeing charges and restrictions on motorists, including the Low Emission Zone (Lez) and the Ultra Low Emission Zone (Ulez).
In March he said: "Every Londoner should be proud of the remarkable progress we've made in improving our air quality since 2016. Together we've cut roadside nitrogen dioxide pollution in half and achieved the lowest annual levels on record."
Transport for London (TfL), which the Mayor of London chairs, confirmed this week that it is proceeding with plans to remove the current £10-a-year exemption which lets owners of battery-powered vehicles drive in London without paying the £15-a-day congestion charge.
A spokesman said the plans were first announced in 2018, highlighting a statement from that year which mainly talked about scrapping exemptions for taxis.
The congestion charge exemption does not apply to the Lez or Ulez schemes.
Confirmation on Tuesday that the zero emission vehicle (ZEV) exemptions will be scrapped were condemned by former Top Gear presenter Quentin Willson, who now campaigns for a faster transition to electric vehicles (EVs).
Mr Willson told The Telegraph that the plans seemed "counterproductive" and that his FairCharge UK group would be writing to the mayor to object to them.
"The cynic in me can't help thinking that this is a revenue based issue," he said. "Why are we stopping it now? And they will say, 'well, it's a congestion issue'. But then surely, it's manifestly better to have cars with zero tailpipe emissions sitting in queues rather than those that are polluting diesel and petrol?
"So to me, it just doesn't work with his campaign to make London a much greener, cleaner air city. It seems completely counterproductive."
Mark Harper, the Conservative transport secretary, said: "Taxing cars even if they're zero emission shows Labour's plan to use motorists as cash cows.
"If they get into government there will be worse to come, with pay-by-mile taxes and fuel duty hikes to help close their £2,000 tax black hole."
Steve Tuckwell, who won the Uxbridge and South Ruislip by-election opposing Labour's "war on motorists", said: "London is the western front in Labour's war on motorists, and only the Conservatives are fighting to defend drivers.
"Sadiq Khan has let Labour's mask slip - scrapping the discounts on electric vehicles makes this clear - Labour wants your car to make as much money as possible for them, whether it's through Ulez or pay-per-mile.
"Don't surrender to Labour's war on motorists; don't let them bring Ulez nationwide. Only Conservative MPs can stand up to this threat."
Motoring groups also hit out at the plans.
Jack Cousens, head of roads policy at the AA said: "The London mayor's decision to force EV drivers to pay the congestion charge from 25 December 2025 onwards may be a Christmas present for him. However, it is a turkey bone in the throat of EV owners who heeded his call to switch to green vehicles and improve air quality in the capital.
"Cabbies and businesses who have made the switch to electric will now face an extra financial hurdle, that ultimately the consumer will pay. Similarly, car-reliant households will now pay more.
"The key question is: has this decision come about because he hasn't made as much money as he thought from the Ulez expansion?"
Simon Williams, the RACs head of policy, said: "This is a somewhat inevitable step as the journey towards zero-emission motoring has progressed considerably since the congestion charge was first introduced, with more than a million EVs now on our roads.
"It was put in place to reduce the number of vehicles using central London roads with a view to improving both air quality and traffic flow. And, now we have the Ulez directly tackling air quality, it's probably only fair that this discount comes to an end as all vehicles, regardless of how they are powered, can create congestion."
A spokesman for Transport for London said: "The congestion charge's cleaner vehicle discount was part of our phased scheme to tackle London's toxic air.
"Ending the cleaner vehicle discount from 25 December 2025 will maintain the effectiveness of the congestion charge, which is in place to manage traffic and congestion in the heart of London.
"The introduction of the London-wide Ulez has resulted in dramatic improvements in air quality and more than 52,000 grants have been approved for Londoners, businesses and charities to switch to cleaner vehicles, with 70,000 fewer non-compliant vehicles now seen driving on London's roads."